Objective: To analyze maternal heart rate (MHR) patterns during labor and evaluate the possibility of misinterpreting MHR as fetal heart rate (FHR).Methods: A retrospective study involving 28 cases of vaginal delivery was undertaken. All 28 cases involved continuous monitoring of FHR and MHR during labor. FHR was obtained from either an external ultrasound transducer or fetal scalp electrode, and MHR by SpO2 monitoring. Representative heart rate tracings of mother and fetus were selected from the 1st and 2nd stages of labor, as well as on delivery, and respective heart rates during interval contractions (as baseline heart rate) and contraction (as periodic change of heart rate) were measured at individual stages. Heart rate changes were compared. A comparison of temporal heart rate change was made between mother and fetus.

Maternal heart rate (MHR) monitoring during labor has some benefits in certain situations. For example, it is well known that maternal arrhythmias are frequently seen during pregnancy in patients with low risk [1] and congenital heart disease [2], and the monitoring of MHR seems to be necessary in such cases. In the case of obstetrical complications such as placental abruption or chorioamnionitis, the monitoring of MHR is useful for the early recognition of altered homeostasis due to bleeding or infection. Apart from morbidity assessment of pregnant women, we often found that the monitoring of MHR was effective in distinguishing it from the fetal heart rate (FHR) during labor.

FHR monitoring is used to assess fetal well-being during antepartum and intrapartum periods. FHR deceleration frequently occurs during labor, especially at the 2nd stage of labor [3]. We occasionally lost the FHR when using trans-abdominal recording. Prolonged deceleration or bradycardia is capable of producing a pathological level of fetal acidemia that occurs in a certain frequency in the low-risk population [4] and may progress without being noticed. Achiron and colleagues showed that MHR tachycardia was obtained from a fetal electrode with a dead fetus and was then interpreted incorrectly [5]. Moreover, MHR acceleration is frequently observed in the 2nd stage of labor [6] and may resemble FHR acceleration. Consequently, there is a possibility of misreading MHR as FHR with a subsequent ominous outcome for the fetus. In order to avoid such misreading, it would be necessary to monitor MHR simultaneously during labor.

Generally, FHRs are rarely misinterpreted. However, as mentioned above, several reports have demonstrated the misinterpretation of MHR as FHR (7, 8, 9). Notwithstanding these findings, there are few articles concerning incorrect obstetric interpretation and MHR characteristics during labor in Japan. Therefore, the purpose of this study was to evaluate MHR patterns during labor and assess the possibility of misinterpreting MHR as FHR.

Materials and Methods

This study was undertaken retrospectively and approved (#2014-104) by the Ethics Committee of our institution. The Perinatal Center of the University of Miyazaki is the tertiary institution in our region and deals mainly with referral cases. We undertook a retrospective study involving 28 consecutive cases of vaginal delivery that monitored FHR and MHR simultaneously during labor between November 1, 2013 and January 31, 2014. We excluded cases with present or past cardiovascular diseases, including maternal arrhythmias. We also excluded cases with epidural anesthesia, which probably affected cardiac output and heart rate due to pain and anxiety. Cases included premature delivery (23 and 34 weeks of gestation), oligohydramnios (n=4), and gestational diabetes mellitus (GDM, n=2). If women had an indication for the use of oxytocin, the increment of oxytocin was conducted according to the Japanese Obstetrics Guideline (JSOG 2011) [10].

We recorded MHR and FHR continuously during labor, and FHR was measured with either an external ultrasound transducer or a fetal scalp electrode (Model FM-50, Atom medical, Inc., Tokyo, Japan). MHR was obtained with a maternal pulse oximetry (SpO2) monitor attached to the equipment. Both heart rates were recorded on the same paper with a speed of 3 cm/min. All information regarding both heart rates was obtained from these tracings. A 10-minute window of FHR and MHR tracings was selected from the 1st (deceleration phase of labor: the time of 6–8 cm dilatation) and initial 2nd stages of labor (10-cm dilatation), as well as just before delivery. In order to avoid the effect of scalp stimulation by internal examination on FHR, we discarded the tracing immediately after the internal examination. Within the 10-minute window of the tracing, the baseline and periodic change of FHR were determined by visual examination. If any accelerations or decelerations occurred in response to uterine contraction, the value of the nadir or summit was determined by visual examination. The greatest FHR decrease or increase was selected as the periodic change of FHR. MHR was also determined visually as with FHR interpretation. In other words, the baseline MHR was roughly defined by increments of 5 beats per minute as for FHR determination during the 10-minute window. If any typical periodic changes of MHR such as acceleration or deceleration occurred in response to uterine contraction, the nadir or summit of the change was determined by visual examination. The greatest MHR decrease or increase was selected as the periodic change of MHR. The individual baseline heart rate and heart rate changes related to uterine contraction were then compared. Additionally, a comparison of temporal heart rate change along with labor was made between mother and fetus. Comparisons of individual baseline heart rate and periodic changes were made using a two-way ANOVA. The comparison of temporal heart rate change between mother and fetus was made using the repeated measure ANOVA. Data are expressed as number, incidence (%), or mean ± SD. Probability values < 0.05 were considered statistically significant.

Results

The maternal age of the study group was 32.0 ± 4.5 years (mean ± standard error of mean) and in the range 23–39. Thirteen women were nulliparous. The average gestational age at delivery was 37.8 ± 3.2 weeks (range: 23–41). Intravenous oxytocin infusion was used in fifteen women because of obstetrical indications that included rupture of membrane, oligohydramnios, failure to progress, or GDM. Four women underwent vacuum extraction, among which two exhibited non-reassuring FHR patterns and two experienced weak pain.

increased significantly on delivery compared to that of the 1st stage (74±11 for 1st stage vs. 86±19 on delivery, p<0.05). On the other hand, except for the 1st stage of labor, FHRs decreased significantly during contraction from baseline values (1st stage: baseline 139±14 and during contraction 137±14 beats/min, not significant; 2nd stage: baseline 137±13 and during contraction 116±23 beats/min, p<0.05; just before delivery: baseline 139±13 and during contraction 83±22 beats/min, p<0.05). It should be noted that there was a significant temporal heart rate change between mother and fetus, and MHR finally exceeded FHR just before delivery (p<0.01, repeated measure ANOVA).

Discussion

Our findings show that the periodic MHR response to uterine contraction was acceleration and the baseline MHR increased toward delivery. The periodic FHR response was deceleration and MHR finally surpassed FHR on delivery. Therefore, the periodic acceleration and elevated baseline of MHR mimicked FHR with acceleration, especially around delivery. It has been reported that NRFHRs occurred more frequently during labor in the high-risk population [11, 12]. Consequently, the misinterpretation of MHR as FHR during labor may not be an uncommon event. In fact, in a report assessing the causes of cerebral palsy from the Japan Council for Quality Health Care in 2014, there were two cases of possible misinterpretation of MHR as FHR during delivery. In both cases, MHR accelerations were misinterpreted as FHR with accelerations, which madethe attending physicians believe the fetus was in a reassuring state, but these cases resulted in severe acidemia and cerebral palsy [13]. Simultaneous FHR and MHR monitoring could prevent these unfortunate events.

Intrapartum FHR monitoring was widely used in routine obstetric practices [14]. However, despite the frequency of electronic FHR monitoring, there is a lack of concern for MHR monitoring during labor in Japan. Moreover, guidelines for obstetrical practice in Japan failed to mention MHR monitoring during labor [10]. In contrast, according to the 2008 National Institute of Child Health and Human Development Workshop Report, MHR monitoring is usually indicated when the FHR pattern is uncertain or similar to MHR because the equipment used to determine FHR may instead be detecting MHR [15]. Our study clearly showed that simultaneous FHR and MHR monitoring could prevent misinterpretation of FHR patterns. Simultaneous MHR monitoring might be recommended as a routine obstetric practice in Japan.

It is well known that MHR increases on uterine contraction [16]. Maternal hemodynamic status greatly changes before and after uterine contraction during parturition. For example, during the latter half of the 1st stage of labor (cervical dilatation ≧8 cm), cardiac output increases from a baseline value of 7.88 l/min to 10.57 l/min during contraction [17]. Similarly, at the end of the 1st ~ 2nd stages of labor (≧8 cm of cervical dilatation), the MHR also increases from a baseline value of 83 beats/min to 96 beats/min [17]. This increment of MHR is mostly due to the increased stroke volume and catecholamine level during uterine contraction [18]. In our study, intravenous oxytocin infusion was used in fifteen women. Oxytocin also affects uterine contraction and peripheral resistance followed by increasing cardiac output and MHR [19]. It has also been reported that MHR can increase to levels comparable to those recorded during heavy physical exercise like running [20]. We therefore have to keep in mind that the increased baseline MHR with phasic accelerations is not an uncommon event, and the pattern of MHR is often close to the FHR pattern during labor.

One of the limitations of this study was the small sample size. However, we thought it was appropriate to reconfirm the fact that MHR changes during labor for small samples because the increased baseline MHR with phasic accelerations is not an uncommon event. Another limitation of this study concerns the methodology used to determine MHR during labor. Visual observation might be an unreliable method to define precise baseline MHR and periodic changes of MHR. However, FHR is usually determined by visual observation of tracings. As such, MHR was also determined by visual observations in this study in a manner similar to that used for FHR interpretation. In conclusion, we demonstrated that the periodic acceleration during contraction and elevated baseline of MHR mimicked FHR with accelerations, especially around delivery. This situation could result in misinterpretation of MHR as FHR. According to our current guidelines for obstetric practice in Japan, MHR monitoring is not indicated during labor. However, we suggest that MHR monitoring should be employed during labor because it allows us to interpret the FHR pattern more accurately and thereby avoid potential fetal complications.

Acknowledgments

We would like to express our gratitude to the midwives of our perinatal center for collecting records.

Disclosure

There is no financial or other relationship that might lead to a conflict of interest.